1. Field of the Invention
The present invention relates to a semiconductor laser and more particularly to such a type of semiconductor laser that has a reduced or no astigmatic difference, or that is capable of generating a laser output focusable outside the laser of having no directivity.
2. Description of the Related Art
In gain waveguide type semiconductor lasers having a narrow stripe structure, the distribution of the light going along the direction parallel with the active layer can be restricted to a horizontal mode (hereinafter referred to as `single lateral mode`) by making the semiconductor laser current restricting stripe narrow in width and by injecting positive carriers to control the gain distribution. With gain waveguide type semiconductor lasers, the carrier intensity becomes large at the center of the stripe. This has the result that the effective refractive index at the center of the stripe is low in comparison to other portions of the stripe which is in turn detrimental to the waveguiding characteristics. In the lateral mode the degree of restriction depends upon the gain, so the perpendicular equiphase surface running in a direction parallel to the active layer cannot be made flat with respect to the direction of light propagation and as will be described below, becomes convex with respect to this direction of propagation. The refractive index waveguide can be improved upon by employing a double hetrodyning structure along the thickness of the active layer so that the perpendicular equiphase surface running in the direction of the thickness of the active layer is flat with respect to the direction of propagation. Equiphase surfaces running parallel with an active layer are basically described by the following equation for a parabolic surface (D. D. Cook and F. R. Nash, Journal of Applied Physics, Vol. 46, No. 4, (1975), reference pp1660); EQU .beta..sub.r.z+a.sub.i.y.sup.2 /2 =a constant (1)
where .beta.r is the complex portion of the propagation constant and k is the wave constant. Also, the direction along the thickness of the active layer is taken as the x axis, the direction along the width is taken as the y axis and the direction of propagation of the light is taken as the z axis. The distribution of the complex dielectric constant within the active layer along the direction running parallel with the active layer is assumed to be the following; EQU e=e.sub.0 -a.sub.2.y.sub.2
where e.sub.0 and a are various complex number parameters expressed by a=a.sub.r +ia.sub.i and e.sub.0 =e.sub.0 +e.sub.0i.
Although equation 1 represents a parabolic surface, when y is in the region of zero the shape can be considered to be cylindrical, and by making the radius of curvature R.sub.m it can be shown that; EQU R.sub.m =.beta..sub.r /(k.a.sub.i).
The resonating end surfaces for reflecting and outputting the light in conventional gain waveguide type semiconductor lasers with a narrow stripe structure are usually flat. So, as can be seen from FIG. 1(A), a point where the width of the laser beam is most constricted, i.e., an imaginary point of a "beam waist" would appear to exist at a point inside the active layer behind the resonating end surface. With regards to this, as can be seen in FIG. 1(B), in the direction of the thickness of the active layer, the point of the beam waist lies at the resonating end surface. The difference between the position parallel with the active region at which the beam waist occurs and the position with respect to the lengthwise direction of the active layer at which the beamwaist occurs is known as the astigmatic difference. It is not possible to produce a highly parallel laser beam or a small wholly circular laser beam spot if there is an astigmatic difference.
The same kind of astigmatic difference which occurs in gain waveguide type semiconductors also occurs in refractive index waveguide type semiconductor lasers when the difference between the effective refractive index in the stripe region and the effective refractive index in the region outside the stripe region is small.
If, however, the optical intensity occurring at the resonating end surfaces of a semiconductor laser having a narrow stripe structure is increased in order to raise the output power, optical damage will sometimes occur at the resonating end surfaces. The width of the stripe is therefore made wider in order to prevent this, thus resulting in a laser having a "wide stripe" structure. Semiconductor lasers having a wide stripe construction are used in applications such as soldering and as a light source for YAG excitation, where high output, small size and low power consumption are required.
Also, the half angle (.theta.//) for a far field image which runs parallel with the active layer is usually from 10 to 20 degrees for a semiconductor laser with a narrow stripe structure. In semiconductor lasers having an embedded hetrodyne structure, even if the refractive index step An within the stripe is made large, the angle .theta.// can still only be made to be between about 30 to 40 degrees. For a semiconductor laser this angle is usually less than about 50 degrees, depending on diffraction effects.
If the kind of semiconductor laser with a stripe structure in which the kind of astigmatic difference described previously takes place is to be used with, for example, a system such as an optical disc system, a lens can be used to focus the light. This, however, makes the diameter of the laser beam spot larger, reduces the light intensity and means that the degree to which the light is parallel is not good.
Technology to correct this astigmatic difference is put forward in, for example, Japanese Laid-Open Patent Publication No. 59-146013. In the semiconductor laser disclosed in this publication, at least one transparent or semi-transparent parallel flat board of fixed thickness is put at the window from which the light beam from the semiconductor laser is outputted, so that the normal vector is inclined at a fixed angle with respect to the optical axis within the semiconductor joining surface of the semiconductor laser so as to correct the astigmatic difference of the light beam. However, as this parallel flat board is necessary to correct the astigmatic difference the structure of the semiconductor laser becomes complicated.
Other technology to correct the astigmatic difference is put forward in, for example, Japanese Laid-Open Patent Publication No. 63-170985. In the semiconductor laser put forward in this publication there are at least a pair of semiconductor end surfaces which act as resonators for the laser. At least one of these end surfaces is lens-shaped and the wavefront of the reflected beam in the vicinity of an end surface is almost flat. However, semiconductor lasers with this kind of structure experience a high degree of optical dispersion loss in the vicinity of the end surface so that the threshold is increased.
As explained previously, the addition of a widestripe structure has been effective in increasing the power output of semiconductor lasers. Where the resonating end surfaces of a semiconductor laser with a widestripe structure are flat, when the output laser beam is focussed by an item such as a lens, the laser beam spot cannot be made to have a single peak, and will become generally rectangular in shape having a length corresponding to the dimensions of the stripe. This kind of beam shape is not suitable for use as a light source for optical discs. Further, it is also not suitable for an SHG excitation light source because the light intensity is low while the light output is as high as one watt, for example.
Also, as mentioned previously, the far field image half angle (.theta.//) for an image in a direction parallel to the active layer is usually less than about 50 degrees for a semiconductor laser with a narrow stripe structure. If this kind of laser beam is a point beam, as, for example, in the case where a signal is sent via a special optical transmission, the positioning of the receiving equipment with respect to the location of the output laser beam is limited. This means that there is a low degree of freedom in positioning the semiconductor laser and the receiving equipment and also means that the designing of the signal transmission equipment is limited.